1. Field of the Invention
The present invention is directed to a method of forming a fracture split in a part, and more specifically, to a method of forming a fracture split in the big end of a connecting rod. The present invention is also directed to a connecting rod having a fracture surface at the big end having both hardened and unhardened portions.
2. Description of the Related Art
Connecting rods are typically attached to the crankshaft of an engine by fabricating the big end of the connecting rod can be separated with a cap part that is separable from the rod part. Typically, the finished cap part and rod part were fabricated separately and then bolted together around a crankshaft pin during assembly. In order to reduce fabrication cost, the cap and rod parts are first forged as a single piece and then broken along a fracture line to form the two individual parts. In some instances, this technique is used to produce fracture surfaces on the rod and cap parts that are uneven. When properly engineered, the uneven fracture surfaces allow the rod and cap parts to maintain a precise, repeatable alignment relative to each other. This alignment is generally critical since it influences the circularity of the resulting crankpin hole when the rod and cap parts are attached together. The uneven fracture surfaces also help to maintain the alignment during use by inhibiting lateral movement between the rod and cap parts. Obtaining and maintaining alignment and circularity of the crankpin hole is important for reducing wear and power loss during engine use.
Various problems can occur during the process of splitting the connecting rod into the rod and cap parts. For instance, when the connecting rod is hardened for better performance, the high loading force necessary to break the big end into the rod and cap parts can deform the big end and decrease the degree of circularity of the surfaces forming the crankpin hole. Also, if a high carbon material is used that is heat treated by quenching, the fracture split surfaces will be in the form of a grain boundary surface that is generally flat, making it difficult to obtain accurate alignment between the rod and cap parts. One method that is used to reduce this problem, which is disclosed in a laid-open Japanese patent publication Sho 58-37310, is to incorporate a plurality of separation assist holes in a zigzag pattern in the area where the big end of the connecting rod is to be split, thereby making the resulting fracture split surfaces macroscopically irregular. However, this approach usually requires that the assist holes be accurate positioned, which increases production cost.
Another problem that can occur during the splitting process is the production of double fractures that result when fracture initiation occurs at more than one point along the break plane. Such double fractures can cause tiny fractions of the fracture split surfaces to come off, mix into engine oil, and damage other components of the engine. Special jigs have been developed for reducing the occurrence of double fractures by concentrating the fracture splitting force to a point on the inside surface of the big end, thus providing a single location for fracture initiation. When this method is used, the jig generally has a complicated shape and requires a high degree of accuracy, which can again increase production costs.
In view of these problems, a method of forming a connecting rod is needed that is relatively inexpensive, but which maintains the circularity of the surfaces forming the crankpin hole and provides fracture split surfaces that are free of double fractures and are sufficiently rough for accurate and repeatable alignment between the rod and cap parts.